Cuckoos don't bother building their own nests - they just lay eggs that perfectly mimic those of other birds and take over their nests. But other birds are wising up, evolving some seriously impressive tricks to spot the cuckoo eggs.

Cuckoos are what's known as brood parasites, meaning they hide their eggs in the nests of other species. To avoid detection, the cuckoos have evolved so that their eggs replicate those of their preferred targets. If the host bird doesn't notice the strange egg in its nest, the newly hatched cuckoo will actually take all the nest for itself, taking the other eggs on its back and dropping them out of the nest.

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To avoid this nasty fate for their offspring, the other birds have evolved a few nifty ways to spot the fakes, which we're only now beginning to fully understand. One of the most intriguing finds is that birds have an extra color-sensitive cell in their retinas, which makes them far more sensitive to ultraviolet wavelengths and allows them to see a far greater range of colors than we humans can. This allows wary birds to detect a counterfeit egg where to our eyes they're all identical.

Fascinatingly, we're actually able to observe different bird species at very different points in their evolutionary war with the cuckoos. For instance, some cuckoos lay their eggs in the nests of the redstart. The blue eggs these cuckoos lay are practically identical to those of the redstarts, and yet they still sometimes get rejected. Compare that with cuckoos who target dunnocks. While those birds lay perfectly blue eggs, their cuckoo invaders just lay white eggs with brown splotches. And yet dunnocks barely ever seem to notice the obvious forgery.

Biologists suspect these more gullible species like the dunnocks are on the same evolutionary path as the redstarts, but they have a long way to go until they evolve the same levels of suspicion. What's remarkable is that the dunnock fakes are so bad and the redstart forgeries so good, and yet cuckoos are still more successful with the former than the latter.

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It speaks to just how radically a species's behavior can be altered by the pressures of natural selection, or it might just be a bit of strategic cooperation on the part of the dunnocks. Biologists have suggested that these birds are willing to tolerate a parasite every so often because they don't want to risk accidentally getting rid of one of their own eggs.